Arrangement for the electrolysis of carbon dioxide

ABSTRACT

An arrangement for the electrolysis of carbon dioxide, includes: an electrolytic cell having an anode and a cathode, wherein the anode and cathode are connected to a voltage supply, wherein the cathode is configured as a gas diffusion electrode to which a gas compartment is attached on a first side and a cathode compartment is attached on a second side; an electrolyte circuit connecting to the electrolytic cell; and a gas supply for supplying carbon dioxide-containing gas into the gas compartment, wherein one or more channels are arranged in the gas compartment, wherein the channels are at least partially in contact with the gas diffusion electrode and are configured for transporting electrolyte fluid passing through the gas diffusion electrode to a lateral region of the gas compartment.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2018/078031 filed 15 Oct. 2018, and claims the benefit thereof. The International Application claims the benefit of German Application No. DE 10 2017 219 766.8 filed 7 Nov. 2017. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to an arrangement for carbon dioxide electrolysis.

BACKGROUND OF INVENTION

The combustion of fossil fuels currently covers about 80% of global energy demand. These combustion processes in 2011 emitted around 34 000 million tonnes of carbon dioxide into the atmosphere globally.

Discussion about the adverse effects of the greenhouse gas CO2 on the climate has led to consideration of reutilization of CO2. CO2 is a strongly bonded molecule and can therefore be reduced back to usable products only with difficulty.

In nature, CO2 is converted to carbohydrates by photosynthesis. This complex process can be reproduced on the industrial scale only with great difficulty. One currently technically feasible route is the electrochemical reduction of CO2. The carbon dioxide is converted here with supply of electrical energy to a product of higher energy value, for example CO, CH4, C2H4 or C1-C4 alcohols. The electrical energy in turn advantageously comes from renewable energy sources such as wind power or photovoltaics.

For electrolysis of CO2, in general, metals are used as catalysts. The type of metal affects the products of the electrolysis. For example, CO2 is reduced virtually exclusively to CO over Ag, Au, Zn and, to a limited degree, over Pd and Ga, whereas a multitude of hydrocarbons are observed as reduction products over copper. As well as pure metals, metal alloys are also of interest, as are mixtures of metal and metal oxide having cocatalytic activity, since these can increase selectivity for a particular hydrocarbon.

In CO2 electrolysis, a gas diffusion electrode can be used as cathode in a similar manner to that in chlor-alkali electrolysis in order to establish a three-phase boundary between the liquid electrolyte, the gaseous CO2 and the solid silver particles. This is done using an electrolysis cell, as also known from fuel cell technology, having two electrolyte chambers, wherein the electrolyte chambers are separated by an ion exchange membrane.

The working electrode is a porous gas diffusion electrode (GDE). It typically comprises a metal mesh, to which a mixture of PTFE, activated carbon, a catalyst and further components has been applied. It has a pore system into which the reactants penetrate and react at the three-phase interfaces.

The counterelectrode is sheet metal coated, for example, with platinum or a mixed iridium oxide. The GDE is in contact with the electrolyte on one side. On the other side it is supplied with CO2. The mode of function of a GDE is known, for example, from EP 297377 A2, EP 2444526 A2 and EP 2410079 A2.

For a continuous process, the electrolysis cell can be operated in flow-by mode. In this mode, the reactant gas diffuses into the pores of the GDE. The reaction in the pores of the GDE forms OH⁻ ions that cause a locally high pH. If the reactant gas CO2 meets this alkaline liquid that additionally also contains alkali metal cations, for example potassium, sparingly soluble carbonates are formed, which precipitate out in salt form and block the pores.

There are various ways of avoiding this, including the utilization of the transpiration of the electrolyte through the GDE. This clears the pores in situ, runs off downward on the gas side of the GDE and is discharged from the cell at the base of the cell together with the unconverted reactant gas and the product gases.

In order to bring about good mixing of the gas in the gas space and hence promote diffusion of the carbon dioxide into the pores of the GDE, flow grids are used. These ensure vortexing and crossmixing and hence promote the mass transfer of reactant and product gases. Furthermore, the flow grids support the GDE, such that it cannot bend.

Although this enables stable long-term operation (>1000 h) of the gas diffusion electrode in CO2 electrolysis, the high liquid content in the gas space reduces the efficiency of the process. Firstly, the liquid film on the GDE makes it difficult for the reactant gases to diffuse into the pores of the GDE. Furthermore, known flow grids in the form of grid structures or knits make it difficult for the transpiration liquid to flow away since it adheres to and accumulates in the flow grids.

SUMMARY OF INVENTION

It is an object of the present invention to specify an improved arrangement for carbon dioxide electrolysis, by which stable long-term operation is enabled with avoidance of the disadvantages mentioned at the outset.

This object is achieved by an arrangement having the features of the independent claim. The dependent claims relate to advantageous configurations of the arrangement.

The arrangement of the invention for carbon dioxide electrolysis comprises an electrolysis cell having an anode and a cathode, where anode and cathode are connected by a power supply, where the cathode is configured as a gas diffusion electrode adjoined on a first side by a gas space and on a second side by a cathode space. The arrangement for carbon dioxide electrolysis further comprises an electrolyte circuit adjoining the electrolysis cell and a gas feed for supply of carbon dioxide-containing gas to the gas space. Finally, the arrangement for carbon dioxide electrolysis comprises one or more channels in the gas space, where the channels at least partly adjoin the gas diffusion electrode and are configured for transport of electrolyte liquid penetrating through the gas diffusion electrode to a side region of the gas space.

The channels advantageously bring about removal of electrolyte that penetrates through the cathode as transpiration liquid and wets the surface of the gas diffusion electrode. If the electrolyte layer on the surface of the gas diffusion electrode is thick enough, the electrolyte begins to run off. The channels guide the electrolyte away to the side and hence reduce the thickness of the electrolyte layer in the region beneath a respective channel. This creates better access for the carbon dioxide to the surface of the gas diffusion electrode and hence enables an improvement in electrolysis efficiency.

Furthermore, the channels, by virtue of their position and arrangement, also ensure flow resistance for the carbon dioxide flowing across the surface of the gas diffusion electrode. This interrupts the laminar flow of the gas and generates vortices. This likewise brings about better utilization of the carbon dioxide present in the gas.

Advantageous configurations of the arrangement of the invention for carbon dioxide electrolysis are apparent from the dependent claims. The embodiment according to the independent claim can be combined here with the features of one of the dependent claims or else with those from multiple dependent claims. Accordingly, the following features may additionally be provided for the arrangement:

-   -   The channels may have an essentially linear configuration and an         oblique arrangement with an angle to the horizontal between 1°         and 30°, especially between 1° and 10°.     -   According to the basic length of the vertical extent of the gas         diffusion electrode, a channel may be present, where the basic         length is between 3 cm and 10 cm. In other words, the distance         between the channels is between 3 cm and 10 cm.     -   The channels are advantageously connected by a common support         structure, where the support structure is spaced apart from the         gas diffusion electrode. This enables anchoring and mechanical         integrity for the channels independently of the cathode since         the channels are retained by the support structure. However, the         support structure does not prevent the access of gas to the         surface of the gas diffusion electrode.     -   The support structure has one or more support pins in mechanical         contact with the surface of the gas diffusion electrode. This         enables better mechanical strength for the support structure.     -   The support pins advantageously have a diameter of less than         1 mm. This achieves the effect that the surface coverage of the         gas diffusion electrode by the support pins is small and hence         the support pins have only a minor influence on the         electrolysis.         -   It is particularly advantageous when at least some of the             support pins have vortexing elements arranged at a distance             from the surface of the gas diffusion electrode of at least             1 mm, especially at least 2 mm. As a result, the gas stream             that otherwise flows across the surface of the gas diffusion             electrode in a largely laminar manner experiences vortexing,             which distinctly improves the access of the carbon dioxide             to the cathode. At the same time, the distance from the             surface of the cathode ensures that electrolyte does not             collect on the surface of the vortexing elements and hence             is prevented from running off. The vortexing elements may             have a corrugated configuration, for example.     -   The support structure, the support pins and/or the channels         advantageously comprise a material having low hydrophobicity,         for example PE. This material advantageously constitutes the         surface of the individual elements, for example as a coating.         Alternatively, it is possible for the support structure, the         support pins and/or the channels to be largely or entirely made         from the material. As a result, the liquid is distributed more         easily over the material surface and the runoff of the liquid is         facilitated.     -   The support structure, the support pins and/or the channels may         also have an electrically conductive material for contacting the         gas diffusion electrode. In the case of the connection of         multiple cells to form what is called a stack, it is possible in         this way, via the flow grid formed from support structure,         support pins, vortexing elements and channels, to electrically         connect a cell unit to the next cell unit.

BRIEF DESCRIPTION OF THE DRAWINGS

An advantageous, but in no way limiting, working example of the invention will now be elucidated in detail with reference to the figures of the drawing. The features are shown here in schematic form. The figures show:

FIG. 1 an electrolysis system for CO2 electrolysis and

FIGS. 2 and 3 a side view and a top view of a flow grid.

DETAILED DESCRIPTION OF INVENTION

The structure of an electrolysis cell 11 shown in schematic form in FIG. 1 is typically suitable for undertaking carbon dioxide electrolysis. This embodiment of the electrolysis cell 11 comprises at least one anode 13 with an adjacent anode space 12, and a cathode 15 and an adjacent cathode space 14. Anode space 12 and cathode space 14 are separated from one another by a membrane 21. The membrane 21 is typically manufactured from a PTFE-based material. According to the electrolyte solution used, another conceivable structure is one without a membrane 21, in which case pH adjustment extends beyond that by the membrane 21.

Anode 13 and cathode 15 are electrically connected by a power supply 22, which is controlled by the control unit 23. The control unit 23 may apply a protective voltage or an operating voltage to the electrodes 13, 15, i.e. the anode 13 and the cathode 15. The anode space 12 of the electrolysis cell 11 shown is equipped with an electrolyte inlet. The anode space 12 depicted likewise comprises an outlet for electrolyte and, for example, oxygen O₂ or another gaseous by-product which is formed at the anode 13 in carbon dioxide electrolysis. The cathode space 14 likewise in each case has at least one product and electrolyte outlet. It is possible here for the overall electrolysis product to be composed of a multitude of electrolysis products.

The electrolysis cell 11 is also executed in a three-chamber structure in which the carbon dioxide CO₂ is introduced into the cathode space 14 via the cathode 15 in the form of a gas diffusion electrode. Gas diffusion electrodes enable contacting of a solid catalyst, a liquid electrolyte and a gaseous electrolysis reactant with one another. For this purpose, for example, the catalyst may be porous and may assume the electrode function, or a porous electrode assumes the catalyst function. The pore system of the electrode is such that the liquid and gaseous phases can equally penetrate into the pore system and may be present simultaneously therein or at its electrically accessible surface. One example of a gas diffusion electrode is an oxygen-depolarized electrode which is used in chlor-alkali electrolysis.

For configuration as a gas diffusion electrode, the cathode 15 in this example comprises a metal mesh to which a mixture of PTFE, activated carbon and a catalyst has been applied. For introduction of the carbon dioxide CO2 into the catholyte circuit, the electrolysis cell 11 comprises a carbon dioxide inlet 24 into the gas space 16. The carbon dioxide in the gas space 16 reaches the cathode 15, where it can penetrate into the porous structure of the cathode 15 and hence react.

In addition, the arrangement 10 comprises an electrolyte circuit 20 by means of which the anode space 12 and the cathode space 14 are supplied with a liquid electrolyte, for example K2SO4, KHCO3, KOH, Cs2SO4, and the electrolyte is returned to a reservoir 19. The electrolyte is circulated in the electrolyte circuit 20 by means of an electrolyte pump 18.

The gas space 16 in the present example comprises an outlet 25 disposed in the base region. The outlet 25 is configured as an opening of sufficient cross section such that both electrolyte that passes through the cathode 15 and carbon dioxide and product gases can pass through the outlet into the connected tube. The outlet 25 leads to an overflow vessel 26. The liquid electrolyte is collected and accumulates in the overflow vessel 26. Carbon dioxide and product gases coming from the gas space 16 are separated from the electrolyte and accumulate above it.

From a point at the upper end of the overflow vessel 26, a further pipe 28 leads to a pump 27, a membrane pump in this working example, and further to the gas feed 17. The pump 27 may also be a piston pump, reciprocating pump, extruder pump or gear pump. Part of the gas feed 17, the gas space 16, the pipe 28 and the overflow vessel 26 together with its connection to the outlet 25 thus collectively form a circuit. By means of the pump 27, the carbon dioxide and product gases present are guided from the overflow vessel 26 back into the gas feed and hence the gas is partly circulated. The volume flow rate of the pump 27 here is distinctly higher than the volume flow rate of new carbon dioxide. As yet unconsumed reactant gas is thus advantageously guided once more past the cathode 15 and has one or more further opportunities to be reduced. Product gases are likewise partly circulated here. The repeated guiding of the carbon dioxide past the cathode 15 increases the efficiency of the conversion.

There is a further connection from the overflow vessel 26 that leads back to the electrolyte circuit 20. This connection begins with an outlet 29 disposed on a side wall of the overflow vessel 26, advantageously close to the base, but not in the base. The outlet 29 is connected to a throttle 30 in the form of a vertical pipe section having a length of 90 cm, for example. The diameter of this pipe section is much greater than that of the feeds to the throttle 30. The feed has, for example, an internal diameter of 4 mm; the pipe section has an internal diameter of 20 mm. The throttle 30 is connected to the electrolyte circuit 20 on the outlet side, i.e. at the upper end of the pipe section.

In the course of operation, the throttle 30 establishes and maintains a pressure differential between the electrolyte circuit 20 connected at the upper end and hence also the cathode space 14 on the one hand, and the overflow vessel 26 and the gas space 16 on the other hand. This pressure differential is between 10 and 100 hPa, meaning that the gas space 16 remains at only a slightly elevated pressure relative to the cathode space 14.

When the electrolysis is started, in spite of the slightly elevated pressure on the gas side, i.e. in the gas space 16, electrolyte is “pumped” out of the catholyte space 14 through the gas diffusion electrode, i.e. the cathode 15, in the direction of gas space 16 on account of the electrical potential applied at the cathode 15. Droplets arise at the surface of the cathode 15 on the side of the gas space 16, which coalesce and collect in shape in the lower region of the cathode 15.

The OH⁻ ions passing through the cathode 15 do cause salt formation together with the carbon dioxide and the alkali metal cations from the electrolyte, but the pressure differential at the cathode 15 is so small that sufficient liquid is purged through the cathode 15 and brings the salt formed into solution, washes it away permanently and transports it out of the gas space 16 into the overflow vessel 26. A further pressure rise that would lead to crystallization of the salt formed is prevented by the throttle 30.

A flow grid 40 is disposed on the gas diffusion electrode. This flow grid 40 is arranged such that the gas flow between the carbon dioxide inlet 24 and the outlet 25 is between the surface of the gas diffusion electrode and a support structure 41 of the flow grid 40. The specific construction of the flow grid 40 is shown in FIG. 2 and FIG. 3.

FIG. 2 shows an enlarged side view of the flow grid 40. The flow grid 40 adjoins the cathode 15 by the right-hand side in FIG. 2. FIG. 3 shows a top view of the flow grid 40 from the side of the cathode 15.

The flow grid 40 comprises a support structure composed of struts or plates that mechanically connects the further elements. The flow grid 40 is concluded on the outside by an essentially rectangular frame 46 that permits gas access and gas exit only at orifices 47 and 48. In the region of the orifices 47, 48, the flow grid 40 has parallel ridges 50 aligned in gas flow direction and one or more baffles 49 to shape the gas flow.

In a middle region of the flow grid 40 is disposed a multitude of support pins 42. The support pins 42 serve to increase the mechanical strength of the flow grid and bring about a fixed minimum distance of the support structure 41 from the surface of the gas diffusion electrode. In the present example, 8 horizontal rows of 9 and 10 support pins 42 in alternation are present. The support pins 42 have a distance from one another of about 6 mm. In other executions of the flow grid 40, therefore, it is also possible for more support pins 42 or fewer support pins 42 to be present, according to the size of the flow grid 40. The distance between the support pins 42 is advantageously between 3 mm and 12 mm. The support pins should cover not more than 10% of the area of the gas diffusion electrode, the coverage advantageously being less than 5%.

At a distance from the surface of the cathode 15 of 1.5 mm, or in another example of 2.5 mm, a vortexing element 43 is disposed on each of the support pins 42. The vortexing elements 43 in the present example are in the form of a flat, essentially rectangular piece of material, but one that has been bent to form a corrugation. The vortexing elements 43 are arranged essentially transverse to the main flow direction of the gas. By virtue of their shape and the remaining flow regions between the vortexing elements 43, the gas flow is made turbulent to a considerable degree, i.e. laminar flow past the gas diffusion electrode is eliminated.

Likewise in the middle region of the flow grid 40, the flow grid 40 also has two channels 44. The channels 44 are secured to multiple support pins 42 in each case and arranged such that they adjoin the surface of the gas diffusion electrode. They are arranged at a small angle from the horizontal of 10°, for example, i.e. are not entirely horizontal. By virtue of their arrangement on the surface of the cathode 15, they take up transpiration liquid, i.e. electrolyte passing through the cathode 15, that runs off downward at the surface of the cathode 15, and transport the liquid to the side by virtue of their inclination. At the side of the flow grid 40, the channels 44 in the frame 41 conclude in a runoff channel 45 that allows the liquid to run off downward to the orifice 48. This achieves the effect that the transpiration liquid wets the surface of the cathode 15 to a lesser degree and hence the entry of gas into the pores of the gas diffusion electrode is hindered to a lesser degree.

Just as in the case of the support pins 42, the number of channels 44 depends on the total size of the flow grid 40 and hence on the size of the cathode 15. The channels are advantageously arranged at a distance from one another of between 3 cm and 10 cm.

In the present example, the flow grid 40 has been manufactured from polyethylene to a significant degree. In other execution variants, it is possible to choose other materials, advantageously having low hydrophobicity. The flow grid 40 may be manufactured wholly or essentially from the material, or the material is applied as a surface coating. By virtue of the low hydrophobicity, the contact angle between the flow grid 40 and the material is minimized, such that the liquid is distributed over the material surface and the best possible runoff is assured. 

1. An arrangement for carbon dioxide electrolysis, comprising an electrolysis cell having an anode and a cathode, where anode and cathode are connected by a power supply, where the cathode is configured as a gas diffusion electrode adjoined on a first side by a gas space and on a second side by a cathode space, an electrolyte circuit adjoining the electrolysis cell, a gas feedfor supply of carbon dioxide-containing gas to the gas space, characterized in that there are one or more channels disposed in the gas space, where the channels at least partly adjoin the gas diffusion electrode and are configured for transport of electrolyte liquid penetrating through the gas diffusion electrode to a side region of the gas space.
 2. The arrangement as claimed in claim 1, in which the channels have an essentially linear configuration and an oblique arrangement with an angle to the horizontal between 1° and 30°.
 3. The arrangement as claimed in claim 1, in which the distance between the channels is between 3 cm and 10 cm.
 4. The arrangement as claimed in claim 1, in which the channels are connected by a common support structure, where the support structure is spaced apart from the gas diffusion electrode.
 5. The arrangement as claimed in claim 1, in which the support structure has one or more support pins in mechanical contact with the surface of the gas diffusion electrode.
 6. The arrangement as claimed in claim 5, in which the support pins have a diameter of less than 1 mm.
 7. The arrangement as claimed in claim 1, in which at least some of the support pins have vortexing elements arranged at a distance from the surface of the gas diffusion electrode of at least 1 mm, especially at least 2 mm.
 8. The arrangement as claimed in claim 7, in which the vortexing elements have a corrugated configuration.
 9. The arrangement as claimed in claim 1, in which the support structure, the support pins and/or the channels comprise a material having low hydrophobicity, for example PE.
 10. The arrangement as claimed in claim 1, having an electrically conductive material for contacting the gas diffusion electrode.
 11. The arrangement as claimed in claim 1, in which the support structure and/or the channels comprise an electrically conductive material for contacting the gas diffusion electrode.
 12. The arrangement as claimed in claim 5, in which the support pins comprise an electrically conductive material for contacting the gas diffusion electrode. 